This invention relates to bioimplantable polymeric articles and more particularly to materials treatment methods for improving the wear resistance and oxidation resistance of such articles.
Advances in biomedical engineering have resulted in numerous polymeric articles having properties suitable for prosthetic use within the body. Polymeric components are widely used in orthopedic surgery, for example, to form articulation surfaces within artificial joints. Ultrahigh molecular weight polyethylene (UHMWPE) is an example of a polymer that is commonly used to form components of artificial joints. Among the properties required of bioimplantable polymeric components, particularly those used in artificial joints, are low friction, biocompatibility, and good mechanical properties such as surface hardness, toughness or mechanical strength, and resistance to wear and creep. Such components must also be sterile before implantation within a patient.
Some polymers and medical devices may be adversely affected by heat sterilization, so this technique is not widely used. Ethylene oxide sterilization, commonly employed for sterilizing other medical articles, may pose health or environmental risks that render this method less desirable. As a result, a preferred method of sterilizing many medical devices, including polymeric components, is by exposure to forms of ionizing radiation such as gamma ray, x-ray or electron beam radiation.
Presently, sterilization by gamma radiation is a preferred by applicant for both cross-linking and sterilizing bioimplantable polymeric components. One potential effect of gamma irradiation is that the gamma rays can initiate chemical reactions within the polymer that can affect the structure, morphology and some mechanical properties of the polymer. During gamma irradiation a variety of chemical species, such as ions, excited molecules, double bonds, oxidation products and free radicals are created within the polymer. Free radicals are believed to be a species generated during gamma radiation that may contribute most to changes in the properties of irradiated polymer.
Once free radicals are formed within a polymer, these species may participate in at least four major types of reactions. The free radicals can undergo a recombination reaction by reacting with hydrogen to eliminate the free radical, by reacting with carbon molecules to create side chains, or both. Free radicals can also initiate or participate in a chain scission reaction that results in a decrease in the molecular weight, and/or change in the density or crystallinity of the polymer, thus causing some mechanical properties of the polymer to degrade. Cross-linking is another reaction in which the free radicals can participate. Finally, the free radicals may remain trapped within the polymer material for an extended time period (e.g., years) even though not initially reacting, and thus remain available to react as conditions dictate.
The presence of oxygen in the polymeric material or in its surrounding environment can contribute to an oxidation reaction in which free radicals and dissolved oxygen react to produce a compound with a carbonyl functional group, resulting in chain scission and the creation of new free radicals. Thus, oxidation can decrease the molecular weight of a polymer (due to such chain scission), which in turn, may contribute to the degradation of its mechanical properties. Since oxygen is ubiquitous in the atmosphere and in biological fluids, this mechanism of degradation may occur whenever there remains a substantial concentration of free radicals in the irradiated polymer. Cross-linking or sterilization of polymer material or components by gamma radiation in air is believed to decrease the wear resistance of polymers due, in part, to such oxidation effects.
Since wear resistance is a key mechanical property for polymeric components that are used in joint prostheses, this problem has now been addressed by techniques such as exposure in oxygen-free environments, by subsequent removal of the affected surface layer, or other processes. Thus, one current practice addresses this problem by irradiating polymeric components in an environment of an inert gas (e.g., argon, helium, nitrogen) to minimize oxidation effects. See, Kurth, M. et al., "Effects of Radiation Sterilization on UHMW-Polyethylene" Antec 87, pp. 1193-1197 (1987); Streicher, R. K., Radiol. Phys. Chem., Vol. 31, Nos. 4-6, pp. 693-698 (1988); Streicher, R. M., "Improving UHMWPE by Ionizing Radiation Cross linking During Sterilization", 17th Annual Meeting of the Society for BioMaterials, p. 181 (1991). Others have used vacuum techniques to help purge an environment of oxygen before conducting gamma radiation sterilization. See, Yong Zhao, et al., J. Appl. Polymer Sci., Vol. 50, pp. 1797-1801 (1993), and Hamilton, U.S. Pat. No. 5,577,368.
Wear resistance is a property of great importance to artificial joint components. Natural friction within a replaced, artificial joint can cause minute particles of debris (e.g., particles from a polymer component) to become dislodged and to migrate within the joint. This phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Wear debris can also lead to osteolysis and bone deterioration. If osteolysis develops around an artificial joint, correction typically requires surgical removal of the diseased tissue and revision of the artificial joint. Thus, to achieve good wear resistance, one must achieve material properties that limit both the amount of wear, and the nature and type of wear debris produced.
Wear resistance depends on many factors, such as the hardness and toughness of the polymer material, which may in turn depend upon specific properties such as the molecular weight of the resins, the degree of cross linking of the material, and the relative size, amount and distribution of regions of amorphous and of crystalline polymer in the microstructure of the finished material. Each of these basic properties may be altered by the application of radiation, heat or chemical agents. Moreover, the fabrication of polymer components generally proceeds from a starting material, or resin, which is provided as either a powder or granular material, or as a consolidated blank, e.g., a sheet or block or preform made from the resin, which must be machined to final form and then be sterilized after the article is fully formed. Heating and pressure, solvation or other factors involved in the consolidation step may influence underlying physical properties, and cross-linking irradiation may be necessary to sufficiently harden material into a wear-resistant solid article, either before or after final shaping. Different forms of irradiation may each have different absorption or interaction characteristics, or generate heat which further affects crystallinity or other properties of the material microstructure. Furthermore, various forms of post-processing may also be required to adjust or overcome properties altered or introduced in the forming and cross-linking stages. Thus, wear resistance depends in a rather complex way upon a number of processing steps which may be employed to treat or physically form the polymer component.
Cross-linking may be primarily effected in part by many of the same processes useful for sterilization, such as heating or irradiation of various types. In general, the radiation dose required to achieve a substantial level of cross-linking may be higher than the dosage needed for sterilization. For example, whereas a dose of 15-25 kGy is effective for sterilization, several times that level may be necessary to effectively cross link a UHMWPE component. At such levels, the evolution of chemical species and free radicals within the polymer may become significant. Moreover, while the level of cross-linking so achieved may result in substantially enhanced hardness or toughness, the irradiation may degrade mechanical properties such as the size or distribution of crystalline and amorphous regions, either immediately due to factors such as heating, or may impair the polymer properties over time as a result of the evolved species as discussed above.
Workers in the field have developed several approaches to processing the polymer material or finished parts in attempts to introduce effective levels of cross-linking while attaining suitable materials properties or rectifying processing damage. These approaches include irradiation in a vacuum, cold irradiation with subsequent melting of bulk material, chemical cross-linking and gas plasma treatment. Each of these techniques appears to enhance at least one parameter. However, the need for multi-step processing, the number of physical processes involved, and the empirical nature of evaluating actual long-term changes in the prosthesis strength and wear properties, all result in a continuing need for processes to enhance polymer strength or wear resistance.
Because excellent wear resistance is a property of such importance for polymer artificial joint components, it would be advantageous to be able to provide highly cross-linked and sterilized polymer components that have an improved and stable wear resistance.
It is thus an object of the invention to provide methods for increasing the wear resistance of a bioimplantable polymer component.
It is also an object to provide sterilization techniques for a medical grade implantable polymer component that impart or preserve important properties of the component.
A further object is to provide bioimplantable polymer component that has improved wear resistance and is less prone to the effects of oxidation.
These and other objects will be apparent to one of ordinary skill in the art upon reading the description that follows.